Temporomandibular disorders (TMD) are often associated with debilitating pain. While both men and women may suffer from TMD pain, the vast majority of those seeking medical attention for relief from TMD pain are women,whose pain is likely to be more severe and last longer than that experienced by men. Indeed, over 80% of those seeking medical attention for relief of TMD pain are women. The molecular basis for the gender difference in the expression and revalence of TMD pain remains unknown. However, epidemiological, clinical and experimental evidence suggests that gonadal hormones, in particular estrogen contributes to this difference. Furthermore, there are several lines of evidence to suggest that a target for estrogen that may contribute to gender differences in TMD pain is temporomandibular joint (TMJ). Importantly, estrogenmay influence the excitability of TMJ afferents in normal tissue as well as the increase in excitability observed in the presence of inflammation. In normal tissue, voltage-and Ca2+-activated channels present in the plasma membrane of the afferent terminal control afferent excitability. Changes in the biophysical properties, expression and/or distribution of these channels may have a profound impact on afferent excitability, however, little is known about the impact of estrogen on ion channels in sensory neurons. Furthermore,while several mechanisms mediating acute actions of inflammatory mediators on sensory neuron shave been identified, consider ablyless is known about mechanis msmediating hyperexcitability in the presence of persistent inflammation and virtually nothing is known about the impact of estrogenon these processes. This is particularly true for joint afferents given the dearth of data on the basic membrane properties of these afferents innervating normal tissue. Therefore, we propose to test the following hypotheses:1) that estrogen increases the sensitivity of the TMJ to noxious stimulation and exacerbates inflammation-induced increases in sensitivity; 2) that estrogen influences the excitability of TMJ neurons through a direct action on these neurons; 3) that estrogen-induced changes in the excitability of TMJ afferents reflects a changes in the expression of voltage-and/or Ca2+ activated channels in these neurons; and 4) that strogen exacerbates inflammation-induced increases in the excitability of TMJ afferents by influencing inflammation-induced changes in the voltage and/or Ca2. activated channels in these neurons. We will test these hypotheses in a series of experiments employing a combination of behavioral, anatomical, electrophysiological and molecular biological techniques on intact male and gonadectomized female rats, as well as gonadectomized female rats receiving estrogen replacement. These experiments are described under 4 SpecificAims.